We will study the factors affecting and limiting oxygen delivery to fetal tissues. This includes studies of placental respiratory gas exchange, studies of fetal cardiac output and organ blood flow, and studies of cellular oxygenation. Specific studies include: 1) stressing the diffusion reserves of the placenta; 2) exploring the relation of placental and fetal organ blood flow to cardiac output; 3) examining the interrelations of fetal cardiac output, blood volume, and extracellular fluid volume; 4) studying the effects of maternal exercise, chronic hypoxia, and chronic hypertension on fetal oxygenation; 5) comparing the effects of hypoxic hypoxia and carbon monoxide hypoxia on the fetus; 6) investigating the relation of oxygen consumption to oxygen tension in the whole fetus and in fetal cells; and, 7) analyzing the effects of chronic low level carbon monoxide exposure on fetal oxygenation.